Wide band amplifier coupling circuits



July 26, 19.49- F. H. M INTOSH WIDE-BAND AHPLIFIER COUPLING CIRCUITS 4Sheets-Sheet 2 Filed D80. 22, 1948 ,1 N VEN TOR.

FRANK H. Me I NTOSH Ju1y26,1949.' F. H. WINTOSH 2,477,014

WIDE-BAND AMPLIFIER COUPLING CIRCUITS Filed Dec. 22, 1948 4 Sheets-SheetZ :JINJL 262 27 7 PM- I 14 INVENTOR.

FRANK H. McINTOSH y 1949- v F. H. MGINTOSH 2,477,074

WIDE-BAND AMPLIFIER COUPLING CIRCUITS Filed Dec. 22, 1948 4 Sheets-Sheets IN V EN TOR.

I FRANK a. McINTOSH July 26, 1949. mm'rosH 2,477,074

WIDE-BAND AMPLIFIER COUPLING CIRCUITS Filed Dec. 22, 14s

4 Sheets-Sheet 4 INVEN TOR.

FRANK H. MC INTOSH Patented July 26, 1949 UNITED STATES PATENT OFFICEWIDE BAND AMPLIFIER COUPLING CIRCUITS.

The present invention relates generally to im- 2 proved audio and videofrequency amplifiers, and to transformers utilizable therein, and moreparticularly to improved class B audio and video frequency electronicamplifiers which introduce extremely slight distortion over a wide bandof frequencies, by utilizing output transformers of novel design,connected in novel relation to the electronic tubes of the amplifier.

The class B amplifier is a push-pull amplifier in which the tubes arebiased approximately to cut-off. One of the tubes, in the normal system,amplifies the positive half cycles of the signal voltage while the otheramplifies the negative half cycles, the output transformer combining theoutputs of the two tubes, to reconstruct a replica of the signalvoltage.

The frequency limits of the conventional audio or video amplifier dependlargely upon the design of the output transformer, loss in amplificationat low frequencies resulting from the low incremental inductance of thetransformer primary, and falling off at high frequencies resulting fromleakage inductance and the various distributed capacities of thetransformer.

In order to obtain a good low frequency response the incremental primaryinductance of the transformer must be high relative to the plateresistances of the tubes used. The primary winding of the transformer,then, should have a large number of turns. At the same time the resonantfrequency of the leakage inductance and secondary capacitance must bebeyond the highest frequency desired to be amplified, so that lowleakage inductance and shunt capacity is essential, if the frequencyresponse of the transformer is to be extended.

The above requirements are mutually conflicting, in various respects.The size of the core of a transformer, i. e., the total iron utilized,is limited by considerations of cost, space and weight requirements.This in turn fixes the total number of turns allotted to the primary andsecondary windings. Decreasing core size and increasing total turns onthe primary winding to retain high primary incremental inductanceincreases leakage inductance and shunt capacity, which in turn, reducesresonant frequency, and hence the high frequency response of thetransformer. In practice, leakage inductance is decreased byinterleaving primary and secondary windings, but this increasesdistributed capacity and so tends to neutralize the benefits obtained.

As a further consideration, high permeability cores must be used, toincrease primary winding 9 Claims. (Cl. 179-171) impedance. Such coresare adversely affected, in respect to the incremental inductance, by D.C. magnetization. Hence the latter must be avoided. The effect ofleakage inductance on class B push-pull amplifiers has been consideredin the literature, and attention is directed particularly to an articleby A. Pen-Tung Sah, in Proceedings of the I. R. E. for November, 1936.Sah points out particularly the deleterious effects of leakageinductance between primary windings of the output transformer of such anamplifier, first, in causing a decreased output, as frequency increases,and second, in introducing finite time constants into the circuit, thuscausing transients which distort the output wave as one of the tubeschanges from a conducting condition to a blocking condition, andvice-versa. The latter effect is the basis of great distortion at thehigher audio frequenmes.

It is a primary object of the present invention to provide improvedpush-pull amplifiers having negligible leakage reactance in their outputtransformers, and hence negligible transient effects during change-overof each tube of the amplifier 25 from conducting to non-conductingcondition.

It is an ancillary object of the invention to provide novel push-pulltransformers having negligible leakage reactance.

It is a further object of the invention. to provide a push-pull wideband transformer of relativelysimple and economical construction, whicheliminates leakage inductance between primary windings of thetransformer.

It is another object of the invention to provide an improved push-pulltransformer comprising bifilar primary windings, and futher to providepush-pull audio amplifiers capable of employing transformer havingbifilar primary windings,

It is, further, an object of the invention to provide a push-pulltransformer, having greater coupling between the secondary winding andthe primary windings than is available in known designs, withoutincreasing detrimental capacity effects, thereby to improve thefrequency response and to enlarge the band width of such transformerswhen employed in amplifiers.

It is still another object of the invention to provide a push-pulltransformer having radically reduced effective distributed capacityacross the primary windings, and to provide a push-pull amplifier foreffectively utilizing a transformer of this character.

It is a further object of the invention to provide a push-pulltransformer of reduced distribpower electronic tubes, wherein isprovided means for maintaining the screen grid of each of the tubes at afixed potential with respect to the associated cathode during operationof the tubes in the amplifier.

It is still another object of the invention to provide a push-pullamplifier arrangement capable of effectively utilizing a transformerhaving substantially zero leakage inductance between its primarywindings, and which requires but a single anode voltage source for allthe tubes of the It is a further object of the invention to providenovel push-pull transformer arrangements which are not bifilarly woundbut which have many of the properties of bifilarly wound transformers,and particularly low or negligible leakage inductance between primarywindings.

It is still another object of the invention to provide a modulatorcapable of supplying large amounts of undistorted power for modulatingcarrier frequency signals.

Briefly described, the various embodiments of the present inventionhereinafter described in detail, and illustrated in the drawings, attainthe objects of the invention by employing bifilar primary windings inthe output transformers to reduce to a negligible value the leakageinductance between these windings. The effect of substantiallyeliminating leakage inductance between primary windings is radically toreduce transients during current cross-over from one to another of thetubes of a push-pull amplifier, these transients being particularlysevere in class B operation. Leakage inductance between the primarywindings and the secondary likewise contributes to these transienteffects, but in reduced degree. Relating the primary windings in themanner stated inherently enables reduction of leakage inductance betweenprimary windings and the secondary winding.

By proper arrangement and connection of the primary windings in thetransformer the equivalent shunt capacity across the primary windings,due to the capacity between windings, which together with leakagereactance and the capacity of the secondary winding determine fallingoff of response at the higher audio frequencies and the high frequencycut-off point of the amplifier, may be similarly reduced, and thewindings may be so related to the electronic tubes of the amplifier thatbut a single anode power supply is required, and that in certain of theembodiments conventional input circuits may be employed.

The conventional mode of reducing leakage inductance consists ofsectionalizing primary and secondary windings and interleaving orinterspersing these. This type of construction is expensive, and whileit succeeds in reducing leakage inductance, results in increasedcapacities.

The ,total capacity of the transformer windings may be reduced byavoiding the necessity for interleaving or pi-winding, in accordancewith the present invention, in order to reduce leakage inductance. Byavoiding the necessity for piwinding, or interleaving, furthermore, thetransformer of the present invention may be arranged more compactly thanpreviously known transformers of the same performance, resulting inreduction of iron requirements, and in a, simplified, more economicallyfabricated core and winding structure.

In the conventional push-pull output transformer for class B amplifiersthe primary Windings of the transformer are connected in series betweenthe plates of the electronic tubes of the amplifier. Accordingly, theprimary windings being closely coupled, the total impedance of theprimary windings is approximately four times the impedance of a singleprimary winding. In accordance with certain embodiments of the presentinvention the primary windings of the output transformer are notconnected in series with each other between the amplifier tubes, but areconnected effectively in parallel. Thereby a reduction in anode terminalto anode terminal impedance of (4), approximately, may be attained.Additionally, each coil, by reason of its bifilar relation to anothercoil, is for the same length of wire and length of coil of double thenumber of layers, resulting in a further decrease of shunt capacity.Reduction of anode to anode impedance of the windings, is, therefore,reflected in a corresponding decrease in anode to anode distributedcapacity across the windings, and therefore in a radical extensionupwards of the cut oif frequency of the amplifier, at its high end.Alternately, more turns may be employed in the primary windings, and theresultant increase of shunt capacity, due to increase in the number ofturns, can be tolerated.

Audio amplifiers constructed in accordance with the present invention,and tested for distortion, have shown less than /a% distortion over theband 20 to 20,000 cycles, the conversion efficiency-of the amplifiertubes remaining above 50% over the band, with an essentially flatresponse over the band of 20 to 200,000 cycles. Nevertheless,transformers constructed in accordance with the present inventioninherently cost less to build than do transformers of the highestquality fabricated in accordance with prior art principles, and requireless space and weigh less than the latter.

Further, no transformers currently available commercially or known to meare capable of attaining the wide frequency response and low wave formdistortion attainable by the present system, regardless of their cost,weight or space.

The above and still further objects, advantages and features of theinvention will become apparent upon consideration of the followingdetailed descriptions of various embodiments of the invention,especially when taken in conjunction with the accompanying drawings,wherein;

Figure 1 is a schematic circuit diagram of an embodiment of theinvention wherein is employed a pair of bifilarly wound primary coils inan output transformer, one of the coils being connected in the cathodecircuit of a vacuum tube of a pushull amplifier, and the remaining coilconnected in the anode circuit of the amplifier;

Figure 2 is a schematic circuit diagram of a further embodiment of theinvention wherein is utilized a transformer having two bifilarly woundprimary coils, each comprising two windings, each bifilarly wound coilhaving one of its windings connected in the cathode circuit and theother in the anode circuit of one of the vacuum tubes of the amplifier;

Figure 3 is a schematic circuit diagram illustratin a modification ofthe system illustrated in Figure 2 of the drawings, wherein the pushpull amplifier utilizes vacuum tubes having screen I grids, each screengrid being maintained at a constant diilerence of potential with respectto its associated cathode during operation of the amplifier;

Figure 4 is a schematic circuit diagram of a modification oi the systemof Figure 2, wherein controllable degeneration is provided in theamplifier;

Figure is a schematic circuit diagram of a variation of the system ofFigure 1 of the drawings, arranged for balanced operation;

' Figure 6 is a schematic circuit diagram of a variation of theembodiment illustrated in Figure 2 of the drawing, wherein the outputtrans- -former is an auto-transformer;

Figure 7 is a view, showing a transformer having bifllarly wound primarywindings for use in, .push-pull amplifiers arranged in accordance withthe invention; and,

Figures 8, 9 and represent variations of the unity coupled transformerof Figure 7.

Referring now more particularly to the drawings and having referenceparticularly to Figure 1 thereof, there is illustrated a push-pullamplifier constructed in accordance with the principles of the presentinvention and utilizing an output transformer arranged in accordancewith the invention.

The amplifier of Figure 1 is illustrated as employing a pair of triodesi, 2 as amplifying electronic devices, the triodes i and 2 beingprovided respectively with grid leaks 3 and s, which are connectedbetween the control electrodes 5 and 6 of the triodes. i and 2,respectively, the midpoint of the grid leak resistors 3 and 4 beinggrounded via a. bias source 1. Driving potential is applied to thecontrol electrodes 5 and 6 from sources conventionally illustrated asgenerators 8, 9, which may be presumed to provide potentials of oppositephases with respect to ground, and of suitable relative magnitudes, thepotentials provided by the sources 8, 9 being applied to the controlelectrodes 5, 6 via coupling condensers l0 and H respectively, theresistors I2 and i 3 representing the internal resistances of sources 8and 9, respectively. The bias established by the bias source I may besuch as to cause operation of the triodes i and 2 to be either as classA, class AB or class B amplifiers, the significance of theclassification being well un derstood in the art, and defined by theInstitute of Radio Engineers in its ofilcial definitions. While thecircuits and structures oi! the present application have wide utility inamplifiers operating in accordance with any one oi the above mentionedclassifications, the invention has primary application to class Bamplifiers, and will be described accordingly as utilized in amplifiersof this class, without intending thereby to limit the scope of theinvention. For the purpose stated, the bias source 1 will be establishedto have a value such as to cut on" the plate current of the triodes iand 2, in the absence of signal voltage applied to the grids thereof.

The input circuits of the triodes l and 2 will, accordingly, be seen tobe completely conventional and to form essentially no part of thepresent invention.

A source of anode voltage is is provided, conventionally illustrated asa battery to simplify the drawings. The negative terminal of source Itis grounded via the lead l5, and the positive terminal of source is isconnected directly via the lead Hi to the anode ll of the triode I, theprimary winding l8 of output transforcer T being connected in thecathode lead or the triode l, intermediate the cathode ll thereoi andthe negative terminal or the potential source II.' The cathode 22 of thetriode 2 is connected directly to ground and a further primary winding2| 0! the output transformer T is connected between the positiveterminal of the potential source I4 and the anode 22 of the triode 2.

The primary windings I! and 2| are wound in biillar manner, orequivalently, as indicated in the schematic circuit diagram, the wiresforming one of the windings being immediately adjacent the wires formingthe other of the windings so that substantially zero leakage inductanceexists as between the windings l8 and 2|.

If it be assumed that a sine wave of potential is applied to the controlelectrodes 5, 6 by the sources 8, 9, the positive halt of the sine wavederiving from source 8 eil'ecting current transfer through the triode I,and the positive hall of the sine wave deriving from source 9 eflectingcurrent transfer through the triode 2, it will be apparent that whilethe positive half of the first mentioned sine wave is applied to thecontrol electrode 5 that the triode 2 is cut oil and that current flowthrough the primary winding It takes place in the direction of the arrow23. On the other hand, while the positive half of the second mentionedsine wave is applied to the control electrode 6 of the triode 2, thetriode l is cut oil and current flow through the primary winding 2!takes place in the direction of the dotted arrow 24 Accordingly, withrespect to the fiux produced in the core 25 of the transformer T,current flow in the windings i8 and 2! is in opposite directions, sothat an alternating magnetic flux is set up in th core 25, and analternating voltage induced in the secondary winding 26 of thetransformer T, for application to the load circuit conventionallyillustrated as a resistance 21.

By virtue of the close coupling existing between the primary windings l8and 2|, the close coupling being brought about by the manner of windingthe primary windings i8 and 2 l, substantially no leakage reactance willexist between these primary windings, and, accordingly, as explained inthe article by Sah, cited hereinbefore, no transient eifects will existduring change over of current carrying function from the triode I to thetriode 2, and vice-versa. At the same time, the direction of thevoltages E existing across both the windings i8 and 2! are always inidentical direction, despite the fact that current ilow in the twowindings is in opposite sense because of the fact that the windingsconduct in alternation and are closely coupled.

If we assume that the triode 2 is cut oif, and the triode l conducting,for example, the winding it induces in the winding 2! a voltagecongruent with its own voltage, and in the same sense in the twowindings, the voltage in winding 2 l, however, being incapable ofcausing current flow in triode 2 because the input voltage applied togrid 6 is now negative in phase and of suificient amplitude with respectto the voltage applied to the anode 22 of triode 2 by winding 2i, toprevent such current flow. Precisely the same argument may be presentedwhen triode 2 is conducting and triode I cut oil.

Furthermore, the terminals 28 and 29 of the primary windings l8 and 2iare directly connected together via the potential source ll, which maybe assumed to have zero impedance, and the total number of turnscontained in the windings it cathode lead of the triode 2.

and 2| and are precisely equal. Accordingly, no A. C. potentialdifference exists between any two adjacent points of the windings I 8and 2|, so that but slight or zero capacitive currents flow betweenadjacent turns of the primary windings l3 and 2|. Such currents as doflow tend to maintain the potentials of adjacent points of the twoprimary windings l8 and 2! identical, and accordingly contribute to theproper functioning of the system.

A condenser C may, if desired, be connected directly from cathode is toanode 22 without al- 'tering the operation of the system essentially,but to assure the equipotential relation between adjacent turns,particularly at the higher frequencies, where some leakage reactancemight conceivably be present due to imperfections of the windingspacings.

It will be noted, upon close analysis, that, the triode i being cathodeloaded and the triode 2 anode loaded, the former is subject todegeneration and the latter is not so subject. The gains of the triodesl and 2 are not equal, for that reason, and the input signals must becompensated accordingly. This feature of the system of Figure 1 detractsfrom its utility, in some degree.

Reference is now made to Figure 2 of the drawings, wherein is discloseda variation of the specific embodiment of my invention illustrated inFigure 1 of the drawings, employing triode vacuum tubes 8 and 2, andhaving signal input circuits duplicating those disclosed in Figure 1,and described in connection with the description of the circuitconnections and operation of the embodiment of my invention thereillustrated, except that the signal sources to. and 3a provide signalsof identical magnitude.

Whereas in the embodiment of my invention illustrated in Figure l of thedrawings a single primary winding is connected in the cathode lead ofthe triode i, and a single primary winding connected in the anode leadof the triode 2, in the system of Figure 2 a more completely balancedarrangement is provided, wherein the primary circuit of the transformerT comprises four windings, one each in the cathode circuits of thetriodes l and 2, and one each in the anode circuits of the triodes l and2. The negative terminal of the anode supply I4 is again grounded, thepositive terminal of one winding 30 being connected via the lead 3i tothe anode 22 of the triode 2, and a second winding 33, in series withwinding 36, being connected via the lead 34 to the anode ii of thetriode 1. Accordingly, the windings 3d and 33 are connected in push-pullrelation to the triodes I and 2, and pass currents in alternation if thetriodes I and 2 are biased for class-,B operation.

A further winding, 35, is connected in the cathode lead of the triode l,and a winding 35, in series with winding 35, similarly connected in theAccordingly, current flow in the winding 35 takes place in phase withcurrent flow in the winding 33, these current flows being additive inrespect to flux production in the core of transformer T. Likewisecurrent flow in the cathode winding 36 is in phase with current flow inthe winding 30, and flux production responsive to the current flow inthe windings 30 and 36 is cophasal in the core of the transformer T.Furthermore, th magnitudes of the currents flowing in the windings 33and 35 are identical and the magnitudes of the currents flowing in thewindings 30 and 36 are identical,

and all the windings 30, 33, 33 and 33 are provided with the same numberof turns. The terminals 31 and 38 of the windings 33 and 38 areconnected together over the extremely low impedance provided by thepotential source I4 and. accordingly, may be assumed to be at the sameA. C. potential. Th phase of the voltages across the windings 33 and 36are identical, for the reasons provided in the explanation of the systemof Figure 1, so that voltage correspondence extends along the lengths ofthe wires forming the windings 35, 36 to the remote terminals thereof.Due to the fact that voltage correspondence exists between every twoadjacent points of the windings 33 and 38, only slight or zerointerwinding current flow takes place by reason of capacities existingbetween the windings. Such current flow as does take place due tocapacitive coupling is, moreover, beneficial rather than detrimentalbecause it tends further to eliminate voltage dider= ences betweenadjacent points of the windings 33, 3d.

The argument presented in the previous paragraph may obviously beduplicated in respect to windings 3t and 35.

Further, the windings 3d and 35 are wound in bifilar or equivalentfashion, as are the windings 33 and 33, so that substantially no leakageinductance exists among the winding pairs 33, 33 and 35, 36.

Since substantially no leakage inductance exists between primarywindings, the efifect of transients due to leakage inductance, whichhave been described in the article by Sah, are completely eliminated inamplifiers constructed in accordance with the arrangement of Figure 2 ofthe drawings. Likewise because interwinding capacity currents areradically reduced, as well as because leakage inductance has beensubstantially eliminated, and for other reasons above provided, the highfrequency resonant point of the transformers is raised by a matter ofoctaves over the resonant frequency of transformers capable of beingconstructed at equivalent cost in accordance with prior art principles.The radical reduction in shunt capacities and leakage inductance,furthermore, eliminates the normally expected reduction of response atthe higher frequencies, so that the amplifier, taken as a whole,provides an extremely fiat response over a very wide band offrequencies. I

Actual examples of amplifiers comprising the invention illustrated inFigure 3 of the drawings have been constructed and found to produce afiat response curve over the band 20-200,000 cycles,

. having less than one-half percent distortion, over the range offrequencies 20 cycles to 20,000 cycles, inclusive, the conversionefiiciency of the amplifier tubes remaining above 50% over this band,and the total amount of copper and iron utilized being equal to or lessthan is employed in high grade transformers of comparable pricepresently commercially available, the performance of the latter beingfar inferior.

The embodiment of my invention illustrated in Figure 3 of the darwingsis substantially similar to that illustrated in Figure 2 of thedrawings, except that the triodes 8-2 are replaced by pentodes dG-Ql,the pentodes being connected in a novel manner to assure high powerconversion emciency.

It will be realized that both the amount and the character of thepotential difference between the cathode and the screen grid of apentode, or a tetrode, are parameters which determine in large part thepower conversion eficiency oi the tube. Since the cathode potentials ofcathode loaded pentode or tetrode tubes vary with respect to ground, itfollows that if the screen potentials are fixed with respect to groundthe cathode to screen potentials will vary, and, in general, the powerconversion efilciency of the tubes will be found to be reduced.

In order to preserve the power conversion eihciency of pentodes andtetrodes when they are connected in cathode loaded circuits, it is usualto connect a condenser between the cathode and the screen grid, and toconnect the screen grid tqp itive anode potential through a resistance.The object of circuit arrangements of this character is the maintenanceof approximately constant potential of suitable value between thecathode and the screen grid. The total potential difference between thecathode and the screen grid approximates the potential differencebetween the positive and negative terminals of the power supply for thetube, less the potential drop due to the current flow in the screendropping resistance and the drop due to D. C. resistance of the cathodeload impedance. It accordingly follows that the size of the screen griddropping resistance varies in inverse proportion to the potentialdifference between the screen grid and the cathode, as the resistance isvaried. However, the minimum size of the screen grid dropping resistanceis limited by the amount of loading imposed on the output circuit due tothis resistance, so that an ideal solution cannot be realized, andmaximum power conversion from pentode and tetrode tubes in cathodeloaded circuits likewise cannot be obtained.

The circuit illustrated in Figure 3 of the drawing provides a solutionto the problem of attaining maximum power conversion from pentode andtetrode tubes in cathode loaded push-pull amplifier circuits, arrangedin accordance with the present invention, the solution consisting inconnecting the screen grid 42 of one pentode 40 directly to the anode 43of the other pentode, and the screen grid 44 of the other pentode 4idirectly to the anode 45 of the first pentode 40. The connection of thescreen grid 42 to the anode 43 implies connection of the screen grid 42to the terminal 45 of the output transformer winding 30, which, as hasbeen explained, in connection with the embodiment of my inventionillustrated in Figure 2 of the drawings, is maintained at the same A. C.potential as is the point 31 of the winding 35. Since the terminal 31 isalways at the cathode potential of the pentode 40, likewise the terminal45 of the winding 30 is maintained at the same A. C. potential as is thecathode of the pentode 40, the D. C. potential exist 'ing between thetwo points being, however, that provided by the potential source l4.Accordingly, as the cathode of the pentode l varies in potential, due tothe presence in the cathode circuit of the current carrying winding 36,the potential of the screen grid 52 varies in precisely similar manner.The difference in potential is thus maintained constant, therebymaintaining maximum power conversion from the pentode.

A precisely similar explanation may be provided in connection with thepentode 4i, this explanation, however, being sufficiently obvious.

It will be further realized that while I have discosed tubes 40 and 4|as pentodes, that precisely the same principles and mode of operationand circuit connections may be employed in conjunction with the use oftetrodes, including beam power tubes, in the circuit of Figure 3.

Reference is now made to Figure 4 of the drawlngs, which illustratesbasically a system of the same character as that illustrated in Figure 2of the drawings, there being added to the latter, however, controllabledegenerative feed-back, still further to reduce the distortion of theamplifier, or in the alternative to necessitate reduced driving signal,as compared with the embodiments of Figures 2 and 3. In the system ofFigure 4 of the drawings, controllable degenerative feed-back is derivedby connecting across the primary windings 35 and 35, which are connectedin the cathode leads of the tricdes I and 2, a resistor 5D, the latterthen having developed across itself a voltage which is a replica of theoutput voltage available at the output of the transformer T. A pair ofvariable taps 5i and 52 are provided, taps 5i and 52 being locatedgenerally at points equidistantly located with respect to cathodes 2Band I9, respectively. Accordingly, by varying the positions of the taps5i and 52 the total feed-back voltage to each of triodes I and 2 may bevaried. The voltage deriving from the tap 5! is applied to controlelectrode 8, via a coupling condenser 53, which is connected to oneterminal of the secondary winding 55 of an input transformer S, which issupplied with exciting voltage via a primary winding 57 excited from thesource of signal voltage A in conventional fashion. The remainingterminal of sec ondary winding 56 is connected to the control electrode6 of the triode 2. While the control electrode 6 is being raised inpotential and the tube 2 is conducting, the potential of the tap 5|decreases in potential due to current flow in resistor 50 in thedirection of the arrow I, the cathode 20 of the triode 2 being then athigher potential than is the cathode III of the triode I. In a similarmanner, the tap 52 introduces a degenerative voltage into the grid ofthe triode I via a coupling condenser 531: which is connected in serieswith one terminal of the secondary winding 58 of the transformer S, theother terminal of winding 58 being connected to the control electrode 5of the triode I. Grid leaks for triodes I and 2 are provided byresistors 59 and 54, respectively connected in serieswith bias source55.

It will be clear, then, that, by moving tap Hi to cathode I9, and tap 52to cathode 2Il, zero degeneration will be introduced into the system,and that degenerative voltages having values as great as twice thosenormally available in the system of Figure 2 may be made available byestablishing tap 5| at cathode I9, and tap 52 at cathode 20.

Reference is now made to Figure 5 of the drawings wherein is illustrateda variation of the system of Figure 1 of the drawings. Specifically, inthe system of Figure 1, a primary winding I8 of a transformer T isconnectedin series with a cathode circuit of a first triode I and afurther primary winding 2|, which is wound in bifilar relation to theprimary winding I8, is connected in series with the anode circuit of afurther triode 2, so that eifectively the triode I is, cathode load ed,while the triode 2 is plate loaded.

In the system of Figure 5 of the drawings the tube I is cathode loadedby means of the primary winding I8 of transformer T. Input signal fromthe secondary 60 of an input transformer S is applied between thecontrol electrode 5 of the tube I and the terminal 51 of winding I8 vialeads 65 and 65. However, the tube 2 is driven in a difierent manner inFigure 5 than is the triode 2 in Figure 1, the tube 2 being effectivelycathode loaded in the system of Figure 5. To this end the primarywinding 2| is connected in the anode circuit of the tube 2, in a similarmanner to the connecions previously described in conjunction with Figurel of the drawings. A further signal is applied in opposite phase to thefirst mentioned input signal via a winding 52 connected between theanode 22 of the tube 2 and the controle electrode 6 of the tube 2 viathe usual block ing condenser C.

Accordingly, the input circuit of tube l sees two alternative voltages,one originating in the primar winding 63 and which is inductivelytransferred to the secondary winding 60, the second constituting adegenerative feed-back voltage deriving from the primary winding E8 ofthe output transformer T by virtue of the connection of the terminal Mof the secondary winding 50 via the lead 65, 66 to the negative terminal6'5 of the primary winding i 8.

The input circuit of tube 2 sees two alternating voltages, oneoriginating in the primary winding 53, which is inductively transferredto the secondary winding 62, the second constituting a degenerativefeed-back voltage, deriving from the primary winding 2| of outputtransformer T, and degenerative by virtue of, the fact that winding M iseffectively in the cathode circuit of tube 2, varying the potential ofcathode 20 with respect to that of anode 22, in one phase, while thepotential of control electrode 6 is being varied in the same phase withrespect to anode 22 by the voltage in secondary 62. Looked at in anotherway, the input voltage for tube 2 consists of the voltages of windings2i and 62 in series. So, when the positive half cycle of voltage inwinding 62 is in the direction of the arrow E, the potential of thecontrol electrode 6 with respect to cathode 20, assuming the latterfixed, increases positively. This results in an increase of platecurrent, which increases the voltage across coil 2!, resulting in avoltage rise across winding 2! in the direction of arrow E. It will beobvious that, as seen from the grid-cathode circuit of tube 2, voltagesE and E are oppositely directed and hence that voltage E isdegenerative.

Additionally, if a screen grid 61 is provided in tube I, this may beconnected directly to anode 22 of tube 2, and will be maintained at aconstant potential with respect to cathode l9 of tube l equal to thevoltage of source H, because the same A.-C. voltages exist at all timeson anode 22 and on cathode 59. If desired a capacitor, C, may beconnected from cathode H to anode 22.

Similarly, a screen grid 68, provided in tube 2, may be connecteddirectly to the positive terminal of source I 4, and will'be maintainedat an A.-C. voltage difference from the potential of cathode 20 equal tothe voltage of source ll, since no impedance exists between screen grid68 and cathode 20.

It will be realized that the tubes land 2 in Figure 5 of the drawingsmay then be triodes,

tetrodes, pentodes, beam power tubes, or the like, as desired, andfurther that in the various embodiments and examples of my invention,illustrated and described herein, the specific character of theelectronic amplifier tubes employed may be selected at will from amongthe-various types available, i. e., triodes, tetrodes, pentrodes, beampower tubes and the like.

The system of Figure 6 represents a simple variation of the system ofFigure 2, demonstrating that, if desired, the bifilar primarywindterminals of the voltage supply.

3% ings 85, 38 may be coupled directly to a load, the transformer "racting as an auto-transformer.

I have disclosed a. variety-of push-pull amphfier systems, employingeach a bifllarly wound transformer, and have illustrated in theschematic circuit diagrams, Figures 1-6, conventionally, bifilarly woundtransformers. I have, however, recited that transformer primary windingsequivalent to bifilar windings may be employed.

Reference is accordingly made to Figures 7-10 inclusive, of thedrawings, wherein is illustrated a plurality of difierent transformerwinding constructions which may be employed in the circuits of Figures1-6, inclusive, Figure 7 illustrating a transformer having four primarywindings, bi-

filarly wound, and associated with a common secondary winding, Figure 8illustrating a possible substitute for the system of Figure 7, employingsuperposed coils in place of bifllarly wound coils,

the transformer of Figure 8 being in some respects equivalent to thetransformer of Figure 7, when the windings are properly connected'in apush-pull amplifier arranged in accordance with the invention. Figures 9and 10 illustrate variants of the transformer of Figure 7 whereinseparate layers of a single coil are incorporated by suitable interlayerconnections in difi'erent primary windings of a push-pull transformer,providing a true approximate equivalent for a bifilarly woundtransformer.

Referring now more specifically to Figure 7 of the drawings, there isillustrated a core E00 of conventional structure having wound thereon acoil i ll! formed of a plurality of bifilarly wound layers 502, thewinding commencing at point 963 and terminating at point we. Leads 6%,M6 are brought out from the commencement point N3 of the bifilarwinding, to which may be connected 3+ and B terminals of a voltagesupply, when the transformer is connected in an amplifier circuit.Similarly from the end of the winding, at point ltd, are brought out twoterminals 807, E08, intended for connection, respectively, to the plateor anode P1 of one amplifier tube of a push-pull amplifier, and thecathode C2 of the remaining tube.

A duplicate coil lie is wound on the same core beside the coil till,having terminals ill and i 52 for connection respectively to 3+ and Band terminals M3, M4 for connection respectively to the cathode Ci ofthe one tube and the anode P2 of the remaining tube;

It will be noted that the respective windings I are wound in oppositewinding senses with respect to the core M0, for reasons explainedhereinabove, and briefly because the coils ill! and H0 are intended toproduce flux in push-pull, or alternately in opposite directions in thecore lB-Il.

Secondary windings M5, M6 are superposed on the primary coils wt andiii), respectively, and are shown connected in series by a lead ill, itbeing understood that parallel connection is equally feasible.

In the broadly or approximately equivalent system of Figure 8, bifilarwindings are dispensed with, and four primary windings are provided,numbered i20, i2l, 522 and H23. The superposed windings Ho and i2! arewound in mutually identical sense, and the superposed windings I22 andH3 in identical sense, the latter two oppositely to the first mentionedtwo windings, and the winding pairs are arranged adjacently on the core.The initial point G24 of winding I20 may be connected to terminal IB-and the terminating point I25 of winding I2I' to terminal 3+, of a platevoltage supply source, by appropriate terminals provided, and theterminal points I24 and I25 being thus Joined by a path of negligibleA.-C. impedance remain at-identical A.-C. potential. The terminal pointI26 of winding I20 and the initial point I2! of winding I2I are arrangedto be in close juxtaposition, and are joined by a condenser Ki, whichserves to maintain the points I26 and iZl at identical A.-C. potentials.

The terminal point I26 may be connected to cathode C2 and the terminalIfil to anode PI.

The coil I22 may be similarly arranged, terminal I 28 being connected to13-, terminal I29 of coil I23 to 3+, and terminals H and I3I joined by acondenser K2, so that the terminals of pair I28, I29 and the terminalsof pair I30, I3i are at identical A.-C. potentials. Terminal I30 may beconnected to cathode CI and terminal I3i'to anode P2, of the tubes ofthe amplifieremploying the transformer. The secondary winding i232 maybe arranged as in the embodiment of Figure 7 of the drawings.

It will be realized that leakage inductance, in the case of theembodiment of my invention illustrated in Figure 8, will be greater thanin the case of the embodiment of Figure 7. However, the transformer ofFigure 8 may conceivably be more economically constructed than thetransformer of Figure 7, and may prove desirable for that reason,despite its relatively poorer performance.

In Figure 9 of the drawings is illustrated a further modification of thesystem of Figure 7, wherein the effect of a bifilar coil is attained bywinding the respective primary windings which are desired to be unitycoupled, in successive layers, and joining the layers thereafter bymeans of suitable leads. Having particular reference to a transformersuitable for use in the amplifier system of Figure 2, for example, thewinding 29 may comprise the winding layers I4I, I42, I43, I44, I45, I46,etc., and the winding 30 the alternate layers I41, I46, I49, I50, II

The terminal point of layer I4I may be connected 1 to B- and its otherend point joined by lead I to an adjacent end point of layer I42, thelayers I and I42 being wound in the same direction and current in eachturn of both layers Ill and I42 flowing in the same sense, to producemutually additive flux in the core. The process of layer interconnectionis continued to the end of the winding, the winding layers I45, I42, I43being thus connected in series. The alternate layers, I41, I48, I49 arelikewise connected in mutual series relation by leads IN, and asecondary winding I63 may be superposed on the primary windings, inconventional fashion. The terminal points of the outermost pair ofadjacent winding layers may then be brought out to anode PI and cathodeC2, respectively.

A similar pair of primary windings, I54 and I65 may be provided on thecore, adjacent to the primary windings 29 and 30, for connection to theanode P2 and the cathode CI, and with the windings 29 and 30 associateda further secondary winding I64, connected in series with secondarywinding I63, it being understood that parallel connection is equallyfeasible.

It will be realized, since the windings are adjacent in alternatelayers, and since the starting points of initial layers HI and I41 areadjacent and interconnected by a path. of low impedance provided by thevoltage source 3+ and B-, that the potentials of adjacent turns of eachpair of layers is ideally at identical A.-C. potential. To compensatefor any departures from ideal conditions, brought about by windingirregularities and the like, I may interconnect the ends of the windingsby means of a large condenser K, which establishes the ends of thewindings at identical A.-C. potential.

Figiu'e 10 illulstrates a winding sequence which approaches that of thesequence provided in the embodiment of Figure 9 of the drawings, thewinding being laid in successive layers, 170 which are left mutuallyunconnected when the coil is wound. The first. fourth, fifth, eighth,ninth, twelfth .layers are connected in series by leads i6I to provideone winding; the second, third, sixth, seventh, tenth, eleventh layersare connected in series by leads I62 to provide the other winding. Theinitial points of the first and second layer may be connectedrespectively to the 23+ and B- terminals of a voltage supply, and thetwo outermost windings (the eleventh and twelfth layers of a twelvelayer winding, for example), connected to the cathode, C2, of one vacuumtube and the anode, Pi, of a further vacuum tube of a push-pullamplifier arranged in accordance with the invention. The latter twoterminals may be connected across a condenser K to assure that the sameA.-C. potential exists at these terminals, as in the transformerarrangements of Figures 8 and 9, inclusive. The upper windings may beduplicated to provide two pairs of biiilarly wound equivalents.

It will further be realized, while the transformers illustrated inFigures 7, 9 and 10 approach relatively closely to the ideal, orbifilarly wound transformer, that the embodiment of Figure 8 is at besta very rough approximation, and, while operative, operates butimperfectly in circuits arranged in accordance with the invention, andis not recommended except in cases where other considerations thanexcellence of performance are primary.

It will further be realized that further variants of the transformersillustrated in Figures 7, 9 and 10 may be resorted to without departingfrom the true scope and spirit of the invention,

which requires the provision of unity coupled transformers, for bestperformance, and which mat, employ any type of unity coupledtransformers having the requisite windings, and which are known or whichmay become known to the art.

Consideration of the system of Figures 2, 3, 4 and 6 of the drawingsWill render evident that each of the tubes of the amplifiers ormodulators disclosed is operated with negative feedback, since' in eachcase a primary winding of an output transformer is connected in acathode lead of a tube and the grid-cathode or input circuit isconnected across the winding. Each of the tubes is, however, also plateloaded so that part of the output of each tube derives from its platecircuit, and part from the cathode circuit. Thereby, I provide apush-pull amplifier which possesses the advantages of both a plateloaded and acathode loaded system, simultaneously, in addition to theother advantages previously disclosed, a feature which has notpreviously been attainable in push-pull amplifiers useful for wide bandamplification of audio or video signals, or the like, to my knowledge.

While I have described various modifications of amplifiers, ormodulators, arranged to employ to advantage output transformers havingbifilarly id wound primary windings, further modifications may bedevised, and re-arrangements and modifications of the modificationsillustrated and described, resorted to, without departing from the truespirit and scope of the inventions, as defined in the appended claims.

In particular it will be realized that the various embodiments of myinvention herein disclosed have particular application, though notexclusive application, to class AB and class B power amplifiers, andtheir variants, and hence to utilization as modulators in varioussystems of modulation, and particularly to class B plate modulators.

What I claim and desire to secure by Letters Patent oi the United Statesis:

l. A push-pull wide band audio frequency amplifier, comprising, a firstelectronic amplifier tube having a first anode, cathode and controlelectrade, a second electronic amplifier tube having a second anode,cathode and control electrode, a source of anode voltage having apositive and a negative terminal, a magnetic core, first and secondprimary output transformer windings of substantially equal inductanceand having each a high impedance at said audio frequencies, arranged inbifllar relation about said core, means vfor connecting said firstwinding between said negative terminal and said first cathode, means forconnecting said second Winding between said positive terminal and saidsecond anode, third and fourth primary output windings of substantiallyequal inductance and having each a high impedance at said audiofrequencies arranged in bifilar relation about said core, meansconnecting said third winding between said negative terminal and saidsecond cathode, means for connecting said fourth winding between saidpositive terminal and said first anode, a secondary winding coupledsubstantially equally to said first, second, third and fourth windings,and a push-pull input circuit for said first and second electronicamplifier tubes, responsive to a wide band audio source for driving saidcontrol electrodes with oppositely phased wide band audio voltages.

2. The combination in accordance with claim 1 which includes means forbiasing said amplifier tubes for class B operation.

3. The combination in accordance with claim 1 wherein said firstamplifier tube comprises a first screen grid, and wherein saidsecondamplifier tube comprises a second screen grid, means for connecting saidfirst screen grid directly to said second anode and means for connectingsaid second screen grid directly to said first anode.

4. A push-pull wide band audio frequency amplifier, comprising, a firstelectronic amplifier tube having a first anode, cathode and controlelectrode, a second electronic amplifier tube having a second anode,cathode and control electrode, a source of anode potential having anegative and a positive terminal, an output transformer having amagnetic core, multiturn primary windings of substantially equalinductance and having high impedance at audio frequencies linking saidcore and coupled to said first and second tubes, said primary windingscomprising at least two' closely coupled windings wound' in identicalwinding sense, means for connecting one of said windings between saidnegative terminal and said first cathode, means for connecting the otherof said windings between said positive terminal and said second anode,said primary windings comprising at least two further closely coupledmultiturn windings of substantially equal inductance and having highimpedance at audio frequencies linking said core and wound in identicalwinding sense, opposite to said first mentioned winding sense, means forconnecting one of said further windings between said positive terminaland said first anode, means for connecting the other of said furtherwindings between said negative terminal and said second cathode, and apush-pull wide band input circuit for applying a wide band of audiofrequencies to said control electrodes in push-pull relation, and meansfor biasing said electronic amplifier tubes for anode current fiow in atleast one of said tubes at all times in response to said signals.

5. I'he combination in accordance with claim 4 wherein said first andsecond electronic amplifier tubes have a first and a second screen grid,respectively, and wherein is provided means for maintaining constantpotential between said first screen grid and said first cathode andbetween said second screen grid and said second cathode, duringoperation of said amplifier.

6. The combination in accordance with claim 4 wherein said means formaintaining constant potential between said first screen grid and saidfirst cathode comprises a direct current connection between said firstscreen grid and said second anode, and wherein said means formaintaining constant potential between said second screen grid and saidsecond cathode comprises a direct current connection between said secondscreen grid and said first anode.

7. A wide band amplifier, comprising, a first amplifier tube having afirst cathode circuit and a first anode circuit, a second amplifier tubehaving a second cathode circuit and a second anode circuit, an outputtransformer having a magnetic core, a first pair of unity coupledprimary windlugs and a second pair of unity coupled primary windings,both linking said core, means for connecting one of said first pair ofwindings in said first anode circuit and the other of said first pair ofwindings in said second cathode circuit, means for connecting one ofsaid second pair of primary windings in said second anode circuit andthe other of said second pair of primary windings in said first cathodecircuit, a load circuit coupled substantially equally to all saidprimary windings, means for biasing said amplifier tubes to providecurrent fiow in at least one of said tubes in response to any finitesignal, and a wide band input circuit connected in push-pull relation tosaid control electrodes for applying said wide band of signals thereto,

8. An amplifier for amplifying a wide band of signals with essentiallyfiat response, comprising, a first electronic amplifier tube having afirst anode, cathode and control electrode, a second electronicamplifier tube having a second anode, cathode and control electrode, asource of anode voltage having a positive and a negative termi- 112.1, amagnetic core, first and second primary output windings-arranged inunity coupled relation about said core, each of said windings having animpedance at the low end of said band, which is of the same order ofmagnitude as the internal resistance of one of said tubes, means forconnecting said first winding between said negative terminal and saidfirst cathode, means for connecting said second winding between saidpositive terminal and said second anode, third and fourth primary outputwindings arranged in unity coupled relation about said core, said thirdand fourth primary output windings each substantially duplicating animpedance one of said first and second primary windings, meansconnecting said third winding between said negative terminal and saidsecond cathode, means for connecting said fourth winding between saidpositive terminal and said first anode, an untuned load circuit coupledto said first, second, third and fourth windings equally, an inputcircuit coupled in push-pull relation to said first and second controlelectrodes for applying to said control electrodes in push-pull relationsaid wide band of signals, and means for biasing said amplifier tubesfor operation with anode current flowing in at least one of saidamplifier tubes at all times in response to said signals.

9. The combination in accordance with claim 8 wherein each of said firstand second amplifier tubes comprises a further control electrode.

18 means for connecting said further control electrode of said firstamplifier tube to said second anode over a path oi negligible impedance,means for connecting said further control electrode of said secondamplifier tube to said first anode over a path of negligible impedance.

FRANK H. MCINTOSH.

REFERENCES crrEn The following references are of record in the file ofthis patent:

UNITED STATES PATENTS 15 Number Name Date 1,791,236 Drake Feb. 3, 19312,187,782 Hardwicl: et a1. Jan. 23, 1940

